The present invention relates to a system for detecting defects in a moving web. Webs, for example textile webs, may often include defects, for example yarn width variations, mispicks, double picks, machine stop marks, and pile irregularities, which are not readily detectable by a human operator. These defects sometimes become apparent only during downstream processing steps, for example dying, or after the fabric is shipped to a customer. According, it is desirable to detect such defects at a relatively early production stage.
Automatic web defect detection systems are known which employ photodetectors disposed with respect to a light source and the moving web to detect defects from the interference between the web and light from the light source. Calibration of such systems so that these photodetectors receive an optimal light signal may sometimes be difficult, particularly as the web or ambient conditions change.